Masks play a major role in the fabrication of integrated circuits. They find application in the optical transfer of geometrical patterns in designing a circuit on the surface of semiconductor wafers. These masks are mainly used in the lithographic stage in which a light-sensitive auxiliary layer, termed photoresist, is patterned by it being exposed through a mask and subsequently developed. The pattern generated in the photoresist is then transferred to the layer underneath by etching.
Since the semiconductor wafers in the fabrication of integrated circuits pass through the lithographic patterning process several times, it is this step that is central in ultimately dictating the costs of semiconductor component production. Since the requirements on the accuracy are very high, this is the factor that makes the lithography highly cost-intensive. Each mask must pass a clearance test before being used for the first time in chip production to verify agreement of the mask content with the circuit design and compliance with the mask specifications while testing the mask for defects.
Apart from this, it is often the case that changes are made to the circuit design during production which thus need to be implemented on the mask. For instance, it may be discovered that better results are achieved with a mask having patterns slightly reduced in size. It is sometimes the case that test patterns on the mask are altered so that they are not translated into the circuits on the wafer, but instead serve to, for example, align the mask relative to the wafer. Making such changes in the course of production harbors the risk, however, of the new mask containing not only the intended changes, but also of incidental deviations finding their way into the system. Although devices are known for testing masks, these are very expensive and thus are normally not available in the field of component production. This is why, hitherto when changes have been made, it is often the case that the new mask is used as a trial on a number of wafers in electrically testing to determine whether the result is satisfactory. This procedure wastes a lot of valuable time, however, until it is assured that the new mask is suitable for fabrication.